Locomotive implantable devices have numerous applications including sensing, imaging, minimally invasive surgery, and research. Many techniques have been used to generate motion, including mechanical solutions and passive magnetic solutions. Power sources dominate the size of existing active implant technologies, and this size constraint (typically in the cm-range) limits the potential for propulsion. Additionally, mechanical propulsion is inherently inefficient at the scale of interest.
Passive locomotion schemes have circumvented the power and efficiency issues, but require large field gradients and usually cannot generate vertical motion. In a passive magnetic propulsion technique, a force is exerted on a small ferromagnetic material with magnetic field gradients. The passive propulsion method typically employs MRI-like systems because the gradient fields must be large and precisely controlled. The gradient must be in the direction of movement, and even MRI systems cannot overcome the force of gravity for devices smaller than roughly 1 mm. The force scales poorly as the size is reduced because it is proportional to the volume of the object. From a practical perspective, generating large field gradients is complicated, and current technology is inadequate.
In addition to the passive method, it is also possible to use mechanical propulsion with active power. Mechanical propulsion is accomplished with a wide variety of techniques. A few possible methods include flagella/motors, pumps, and acoustic streaming. These designs typically suffer from low conversion efficiency from input power to thrust, especially as the Reynolds number decreases. There are losses associated with the conversion from electrical power to mechanical motion, and more loss associated with the conversion from mechanical motion to forward thrust. As a result of the low efficiency, a fairly substantial amount of power is required, and the power source dominates the size making it difficult to miniaturize.